1. Field of the Invention
The present invention relates to a motor control circuit and apparatus which may be adapted, for example, to provide proportional positioning of a valve spool or variable displacement pump in response to an electrical command signal. More specifically, the present invention relates to a motor control circuit which causes a motor shaft to oscillate between an incremental rotational position nearest above and nearest below a commanded average rotational position identified by an electrical command signal.
2. Background of the Prior Art
Electronic remote operation of valves and variable volume piston pumps has been used for many years. Traditionally, electrical positioning of valve spools has been obtained by providing a pilot pressure line to one end of a spool from a pressure control pilot valve that electrically varies the pressure exiting the pilot valve. The force on the one end of the spool acts against a spring force in the opposite direction thus making the spool position proportional to the pressure. Alternatively, two pilot pressures operating on opposite ends of the spool may be used so that the net pressure force acts against a centering spring.
A number of methods have been used for electronically varying the pressure on the end of the spool. For example, the use of a permanent magnet torque motor to operate a nozzle flapper pilot stage or a jet deflector pilot stage has been practiced. Yet another method utilizes two proportional or pulse width modulated solenoids, each of which controls the pressure on an end of the spool. Finally, the use of a push-pull arrangement of solenoids to mechanically move a small pilot spool which directs pressure to opposite ends of the main valve spool has been used
Each of these methods provides a theoretical balance of net pressure forces generated by the pilot valves on opposite end areas of the spool against a spring force that acts to center the spool. In practice, however, this pressure-spring balance is greatly influenced by fluid flow forces, friction of the spool in the housing, the drag of seals and frictional forces of associated components.
As a result, the positioning forces of the spring and pressures must be made very high compared to the frictional and flow forces so that the basic spool position is determined by positioning forces (pressure forces versus spring forces) and not the unwanted forces (friction and flow forces). Because of constraints in springs and space limitations, the compromises which must be made result in devices which typically have 15% hysteresis (due to friction) and 25% non-linearity (due to flow forces).
To avoid undesirable uncertainties in spool position, various schemes to feed back the spool position to the pilot valve have been employed to increase positional pressure gain and thus reduce hysteresis and non-linearity. Both mechanical and electrical feedback devices are currently in use.
Mechanical feedback operates by providing some sort of spring force feedback of the main valve spool position to the pilot stage. Because of pilot stage output force limits the magnitude of this spring force this method does not provide much of a safety margin. Additionally, because the spring also utilizes some or most of the force required in the pilot stage, the pilot stage must be made larger (higher output force).
Electrical feedback may be accomplished with a potentiometer, strain gage, LVDT or other position transducer used to yield an electrical signal proportional to spool position. This type of feedback greatly increases the pressure gain to position the spool but has a major limitation in that the loss of the feedback signal for even a short time will force the main valve spool to its full flow position and electrical position transducers add to the overall cost of the system.
In summary, the existing devices and methods utilized for the electrical positioning of valve spools which do not utilize feedback are plagued with problems of hysteresis and repeatability. Those devices and methods which use feedback of main valve spool position may be unacceptable for many applications because of a lack of fail safe back-up systems to address the problems associated with the possible loss of the feedback signal.